The government was warned today not to use the excuse of "deliberate strategic ambiguity" as a cover for lowering the threshold on launching nuclear weapons in a crisis.

The Commons defence committee also challenged the operational significance of Downing Street's promise to cut Britain's military nuclear stockpile by up to 20%, when a new generation of missile submarines would carry the same number of warheads as their predecessors.

The MPs' report on the future of the UK's deterrent force is published a week before a parliamentary vote on building three missile boats to replace the current Vanguard-class submarines.

Ministers expect a substantial Labour back-bench revolt over the issue, but are confident of winning with the support of the Tories.

While accepting most of the arguments for maintaining the strategic deterrent, the MPs said they remained concerned over the refusal to define what would constitute the "extreme circumstances in defence of the UK's vital interests", cited by Ministers as a reason to use it in anger.

Des Browne, the Defence Secretary, refused to be drawn, claiming that "keeping the enemy guessing" was an important element of the deterrent process.

Whitehall sources say the committee members are worried that the rules have been changed quietly under the cloak of secrecy to allow a possible first strike against a perceived threat from states such as Iran.

UK policy through most of the Cold War was that warheads would be launched only in retaliation for a direct and devastating attack on Britain or on its forces or key interests abroad.

Mr Browne assured the committee that Britain's nuclear weapons were not designed or intended for warfighting operations.

MPs now want an explanation of why, if that is true, government plans include the declared option of building sub-strategic warheads as a deterrent against smaller threats from rogue states.

The need for lower-yield warheads has been policy since the 1998 Defence Review and all of the UK's deterrent boats have gone to sea since then carrying smaller tactical warheads on at least one missile to allow the flexibility of a limited strike.

While the government has pledged to reduce the overall warhead stockpile to fewer than 160 from about 200 now, new submarines are still expected to go on patrol with 48 warheads available for launch - the same number carried by the Trident boats.

A Ministry of Defence spokesman said: "We have made clear repeatedly that we would only ever consider use of our deterrent in extreme circumstances of self-defence. There is nothing in the White Paper on the future deterrent to suggest lowering the nuclear threshold."

Alan Mackinnon, chairman of Scottish CND, added: "There are far too many unanswered questions. Labour MPs should not be rushed into signing a blank cheque for Trident or its replacement."